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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

204                                ALASKAN GLACIER STUDIES
expanded piedmont bulbs; but the attack of the sea water and the rapid dispersal of ice that falls into the fiord from the glacier front, succeeds in truncating the bulbs, so that they are only partially developed. In those portions of the ice front from which icebergs are discharged the conditions are similar to those of Nunatak Glacier, and the remarks made about the deposits there apply in all essential particulars here also. Along the margins, however, the conditions are different, for in their lateral spreading both the Turner and the Hubbard Glaciers have extended so far as to give rise to small areas of stagnant or partially stagnant ice on either margin. Along these less active areas marginal streams flow and each has built a small alluvial fan.
A number of glaciers, including several of small size, expand at the mouths of their valleys giving rise to piedmont bulbs, or bulb glaciers. These are sometimes in larger valleys, like Variegated Glacier in the Russell Fiord trough, but more commonly are at the mountain front, like Galiano Glacier. Where two or more glaciers descend from neighboring valleys, and expand sufficiently, the piedmont bulbs may coalesce and form a piedmont glacier, as is illustrated by the Atrevida-Lucia, and, far better, by the Malaspina Glacier, formed by the union of several large glacier bulbs, and the type case of the piedmont glacier, made known to us by Russell's researches. The piedmont glaciers and the piedmont bulbs have many peculiar and interesting features,, including marginal deposits of varied character, ablation moraines, buried ice blocks,, and outwash gravel deposits, fully discussed in later pages.
SPECIAL PROBLEMS
Among the glacial phenomena some warrant more detailed discussion. These relate mainly to the piedmont condition, to some of the phenomena associated with other glaciers, and to the work of the formerly more extensive ice tongues.
The Piedmont Bulbs. The phenomenon of spreading of glaciers on emergence from valleys is common in the Alaskan region. Indeed, it is universal wherever the glaciers extend beyond the valleys which confine their upper portions, whether the escape be into a broader valley, as in the Variegated Glacier, or beyond tJie mountain front, as in the Atrevida. This spreading to the piedmont bulb condition is evidently due to lateral flowage where an ice supply is released from lateral confinement, and it bears a very close resemblance to the flowage of wax or other viscous substance. We interpret it as a phenomenon of viscous flowage of unconfined ice.
In all the cases which we have studied, the phenomenon of the piedmont bulb expansion1 is the product of a previous stage. We cannot, therefore, give a description of the piedmont condition at the time of formation. We infer, however, that the ice of the entire piedmont area was then in motion, though with great diminution in velocity* toward the periphery, reaching stagnation and perhaps being almost motionless around the margin, and especially in the lateral margins. Nor were we able to describe the ice currents in such a bulb. Judging from the great morainic swirls on the Malaspina Glacier, and the crescentic banded moraines on the Variegated Glacier bulb, the ice currents are complex and peculiar and a determination of their nature would be a matter of great .interest. The currents seem to give rise to a broadening and spreading of medial and lateral moraine bands; but our knowledge of their nature and behavior is so slight that we are unable at present to go farther than we have done in our description and interpretation of the morainic phenomena of the Variegated Glacier.